Soiling measurement system for photovoltaic arrays

ABSTRACT

A system for measuring the power or energy loss in a photovoltaic array due to soiling, which is the accumulation of dust, dirt, and/or other contaminants on the surfaces of photovoltaic modules, comprising: a pair of photovoltaic reference devices placed within or near the photovoltaic array and co-planar to the modules comprising the array, wherein one reference device is a module or cell similar to those of the array and is allowed to accumulate soiling at the natural rate, and wherein the second reference device is a module or a cell and is maintained clean; and a measurement and control unit which measures and compares the electrical outputs of the soiled reference device and the clean reference device in order to determine the fraction of power lost by the soiled reference module due to soiling.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application:

is a continuation-in-part of U.S. Non-Provisional patent applicationSer. No. 15/482,291, filed Apr. 7, 2017, now U.S. Pat. No. 9,800,202;

which is a continuation of U.S. Non-Provisional patent application Ser.No. 15/346,495, filed Nov. 8, 2016;

which is a continuation-in-part of U.S. Non-Provisional patentapplication Ser. No. 14/381,165, filed Aug. 26, 2014, now U.S. Pat. No.9,564,853;

which is a national stage entry of PCT Patent Application Ser. No.PCT/US13/71,315;

and claims priority to U.S. Provisional Patent Application Ser. No.61/876,134, filed Sep. 10, 2013;

and claims priority to U.S. Provisional Patent Application Ser. No.61/728,898, filed Nov. 21, 2012.

Each of the above are incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to a system for measuring power or energy loss insolar power plants due to accumulation of dust, dirt, and othercontaminants (or “soiling”) on photovoltaic (PV) modules.

BACKGROUND OF THE INVENTION

Arrays of photovoltaic (PV) modules, also known as solar panels, areused in solar power installations for converting sunlight toelectricity. Such installations range from small rooftop systems onresidential or commercial buildings to large utility-scale facilitiesincluding thousands or millions of PV modules. Collectively, we refer tothese as “solar power plants.”

Frequently solar power plants employ performance monitoring systems,especially in large commercial or utility-scale facilities. Thesesystems monitor output power and meteorological conditions, allowingplant developers and owners to confirm that performance meetsexpectations and allowing system operators to identify fault conditionsor underperforming equipment.

Among the most significant meteorological conditions affecting solarpower plant performance are the solar irradiance at the site, theambient temperature at the site, and the accumulation of dust and dirtor other contaminants on the PV modules.

Accumulation of dust, dirt, or other contaminants is known as “soiling”.Soiling reduces the solar irradiance transmitted to the active area ofthe PV modules, thus reducing power output. Soiling levels of PV modulesaccumulate in between rainfalls or scheduled cleanings of the PV array.Losses often accumulate at rates of approximately 0.1% to 0.3%additional power loss per day, depending on local conditions at thesite, and may accumulate up to levels of ˜5-20% power loss (or greater)during extended periods without rainfall.

Developers, owners, and operators of solar power plants wish to quantifypower losses due to soiling in order to confirm the underlyingperformance of the power plant and also to determine if and whencleaning the PV array provides an economic payback.

In addition, prior to construction of a solar power plant, developers,owners, and operators often wish to assess conditions at a site for aprospective solar power plant, in order to determine how siteconditions—including soiling—may affect the performance of theprospective solar power plant.

Losses due to soiling may be quantified for an operating solar powerplant by monitoring the overall efficiency of the solar power plantversus expectations based on meteorological conditions, for example asdiscussed in Kimber, et al, “The Effect of Soiling on LargeGrid-Connected Photovoltaic Systems in California and the SouthwestRegion of the United States,” Conference Record of the 2006 IEEE 4thWorld Conference on Photovoltaic Energy Conversion, vol. 2, pp.2391-2395, May 2006. However, this method assumes that all efficiencylosses are due to environmental factors and that there is no underlyingfault or degradation in the solar power plant performance that wouldalso result in efficiency loss. This method also does not apply toassessing soiling-related power losses from prospective solar powerplants prior to construction.

Another method quantifies soiling-related losses by comparing thetemperature-corrected short-circuit current of two identical testmodules representative of those in the PV array, one of which (the“soiled” module) is allowed to soil at the natural rate of the PV arrayand the other of which (the “clean” module) is kept clean, througheither manual or automatic washing. This method utilizes the principlesthat the temperature-corrected short-circuit current of a PV module isproportional to the irradiance reaching the module, and that the powerproduced is a known function of irradiance. The short-circuit current istypically measured by means of the voltage drop across a verylow-resistance shunt resistor connected between the module terminals.This method is described, for example, by R. Hammond, et al, “Effects ofSoiling on PV Module and Radiometer Performance,” Proceedings of26.sup.th IEEE Photovoltaics Specialist Conference (PVSC), Anaheim,Calif., Sep. 30-Oct. 3, 1997; Miguel Garcia, et al, “Soiling and OtherOptical Losses in Solar-Tracking PV Plants in Navarra,” Progress inPhotovoltaics: Research And Applications, vol. 19, pp. 211-217, 2011;Caron, et al, “Direct Monitoring of Energy Lost Due to Soiling on FirstSolar Modules in California,” Proceedings of the 38^(th) IEEEPhotovoltaic Specialists Conference (PVSC), Austin, Tex., Jun. 3-8,2012; and Caron, et al, “Direct Monitoring of Energy Lost Due to Soilingon First Solar Modules in California,” IEEE Journal of Photovoltaics,vol. PP, no. 99, pp. 1-5, Oct. 24, 2012.

However, since this method estimates power loss from measurements ofshort-circuit current, i.e. from effective irradiance reaching themodule, it will yield inaccurate results in certain situations,principally when the soiling is accumulated non-uniformly across thesurfaces of the modules. Such non-uniform distributions of soilingfrequently occur due to the influences of module orientation, wind,rain, and gravity, often resulting in predominant soiling across oneedge or another localized region of the modules, as illustrated inphotographs of soiled photovoltaic arrays shown in FIG. 1A and FIG. 1B.Various distributions of soiling can lead to different ratios betweenthe modules' short-circuit current and output power at a givenirradiance. This is explained in more detail below.

Furthermore, the method described above requires keeping one of the twoidentical PV modules clean, with cleaning preferably performed daily.The cleaning can be performed manually, although this createssignificant labor expenses. Alternatively, an automated system can beused to keep the clean module washed. However, this may requiresignificant water usage. As water supplies are not typically availableat solar power plant installations, especially in remote or desertsites, large storage tanks may be required to operate such equipment.This is expensive and creates maintenance problems.

BRIEF SUMMARY OF THE INVENTION

The disclosed subject matter provides a system for measuring thefraction of power or energy lost due to PV module soiling in an actualsolar power plant or a prospective solar power plant to be constructedat or near the site of the measuring system. The disclosed subjectmatter addresses the shortcomings of existing technology and practicesby providing more accurate measurements and more economical apparatuswith minimized maintenance requirements.

The system comprises a soiled reference module 100 and a clean referencedevice which may be either a clean reference module 101 or a cleanreference cell 102. The pair of reference devices (100 and 101, or 100and 102) are coupled to a measurement and control unit 104 and aremounted on a mounting rack 106, substantially co-planar with each otherand substantially co-planar with the modules of the actual orprospective solar power plant. The system is placed outdoors at the siteof the actual or prospective solar power plant, and, in the case of anactual solar power plant, is located substantially near or within thephotovoltaic array 10 of the solar power plant as depicted in FIG. 2.

The soiled reference module 100 is of substantially the same type and ismounted in substantially the same way as the PV array 10 modules used inthe actual or prospective solar power plant, and is therefore allowed toaccumulate soiling in the same manner and at the same rate.

In one embodiment, the clean reference device is a clean reference cell102, which is relatively small compared to a full-size module.

In one embodiment, the clean reference device (101 or 102) isperiodically cleaned with an automated system, which may includehigh-pressure spraying with a cleaning fluid 306, cleaning withmechanical action, cleaning with a pressurized gas flow, or combinationsthereof. In one embodiment, the system, apparatus or method includes acollection tray, capable of capturing the dispensed cleaning fluid forre-use.

The measurement and control unit 104 measures the soiled referencemodule 100 temperature and electrical output parameters, including boththe short-circuit current and maximum power, as well as the solarirradiance detected by the clean reference cell 102 (or clean referencemodule 101). These quantities are analyzed, either by the measurementand control unit 104 or by a remote computing system (not shown) inorder to determine the expected electrical output power of the soiledreference module 100 at the given conditions of temperature andirradiance in the absence of soiling, and there from to determine thefraction of output power from the soiled reference module 100 that islost due to soiling, or, equivalently, the fraction of power actuallygenerated in the soiled state compared to the power which could begenerated in the absence of soiling.

Particular objects of the disclosed subject matter include: measuringthe soiling-related power loss and/or the rate-of-change ofsoiling-related power loss over time; measuring such losses accuratelyfor modules of the same type used in the actual or prospective solarpower plant; measuring such losses accurately for both uniform andnon-uniform distribution of soiling across a module; and providing aneconomical, automated measuring system requiring minimal maintenance.Any particular embodiment does not necessarily contain all the precedingobjects, and one or more of the preceding objects may be added orremoved from an embodiment and stay within the scope of this disclosure.

A system according to the disclosed subject matter achieves theaforementioned objects through the following major aspects whichtogether improve upon existing technology and practices: the soiledreference module 100 is substantially identical to those in the PV array10 and mounted in substantially the same manner, therefore accumulatingsoiling-related power losses in a manner representative of the PV array10; both the short-circuit current and maximum power output of thesoiled reference module 100 can be measured, accounting for the effectsof possible non-uniform soiling; measurements of the soiled referencemodule 100 are analyzed together with measurements from a cleanreference device (101 or 102) allowing discrimination of soiling-inducedlosses from other losses; in one embodiment a relatively small cleanreference cell 102 is used as the clean reference device, and, in oneembodiment, the clean reference cell 102 is cleaned with an automatedsystem, which requires minimal cleaning fluid usage due to therelatively small size of the clean reference cell 102.

In one embodiment, the reference devices (100, 101, and/or 102) can bemaintained in a defined state in between measurements, thus maximizingtheir expected life span. Examples of this state include: short-circuitstate, an open-circuit state, and a maximum power state.

In one embodiment, the system calculates and measures a current-voltagerelationship from which are determined the short-circuit current, anopen-circuit voltage, and/or a maximum power output.

One embodiment of the present disclosure includes a system, apparatusand method for determining and responding to non-uniform soiling ofsystem, apparatus or photovoltaic array.

One embodiment of the present disclosure includes a system, apparatusand method for responding to a difference in angular alignment betweenthat of the clean and soiled reference devices, and/or the apparatus andthe photovoltaic array.

These and other aspects of the disclosed subject matter, as well asadditional novel features, will be apparent from the descriptionprovided herein. The intent of this summary is not to be a comprehensivedescription of the subject matter, but rather to provide an overview ofsome of the subject matter's functionality. Other systems, methods,features and advantages here provided will become apparent to one withskill in the art upon examination of the following FIGURES and detaileddescription. It is intended that all such additional systems, methods,features and advantages that are included within this description bewithin the scope of the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features believed characteristic of the disclosed subjectmatter will be set forth in the claims. The disclosed subject matteritself, however, as well as a preferred mode of use, further objectives,and advantages thereof, will best be understood by reference to thefollowing detailed description of illustrative embodiments when read inconjunction with the accompanying drawings, wherein:

FIG. 1A and FIG. 1B illustrate soiling accumulation on photovoltaicarrays, with examples that particularly demonstrate spatiallynon-uniform soiling accumulation. The photograph in FIG. 1A is extractedfrom E. Lorenzo, R. Moreton, and I. Luque, “Dust effects on PV arrayperformance: in-field observations with non-uniform patterns,” Progressin Photovoltaics: Research and Applications, 2013, which is incorporatedherein by reference, wherein i) illustrates a soiled PV array in a solarpark, ii) is a detail of i), and iii) and iv) illustrate soiled rooftopphotovoltaic arrays. FIG. 1B is extracted from F. Brill, “EnviroPoliticsBlog: PSEG building solar farms—and not just in New Jersey,” 16 Nov.2012, (Online), Available:http://enviropoliticsblog.blogspot.com/2012/11/pseg-building-solar-farms—and-not-just.html#.UagS95xXqdI(Accessed: 31 May 2013), which is incorporated herein by reference, anddepicts a soiled PV array with modules mounted on a one-axis trackingsystem and spatially non-uniform accumulation of soiling.

FIG. 2 depicts a soiling measurement system according to the disclosedsubject matter, located substantially near or within a photovoltaicarray to be monitored.

FIG. 3 depicts the effects of uniform and non-uniform soiling on theshort-circuit current and maximum power of a PV module, using simulatedI-V curves of a 72-cell PV module under various conditions. Open circlesrepresent the maximum power point of each curve.

FIG. 4A and FIG. 4B depict alternate embodiments of the disclosedsubject matter, wherein soiling is determined from soiled referencemodule 100 and wherein solar irradiance for reference in soilingdetermination is measured with either A) a clean reference module 101,substantially identical to soiled reference module 100, and which iscleaned either manually or automatically or B) a clean reference cell102 which may be automatically cleaned by wash unit 108 of the disclosedsubject matter.

FIG. 5 depicts one embodiment of a system according to the disclosedsubject matter.

FIG. 6 depicts a transient measurement of an I-V curve in one embodimentof the measurement and control unit 104.

FIG. 7 depicts an embodiment of the I-V sweep circuit of the measurementand control unit 104.

FIG. 8 depicts an embodiment of the wash unit 108.

FIG. 9 depicts an embodiment of the clean reference cell 102.

In the figures, like elements should be understood to represent likeelements, even though reference labels may be omitted on some instancesof a repeated element for simplicity.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The disclosed subject matter provides a system for measuring thefraction of power lost due to PV module soiling in an actual solar powerplant or a prospective solar power plant to be constructed at or nearthe site of the measuring system.

Although described with particular reference to a soiling measurementsystem for solar power plants, those with skill in the arts willrecognize that the disclosed embodiments have relevance to a widevariety of areas in addition to those specific examples described below.

All references, including publications, patent applications, andpatents, cited herein are hereby incorporated by reference to the sameextent as if each reference were individually and specifically indicatedto be incorporated by reference and were set forth in its entiretyherein. Provided, however, to the extent there exists a conflict betweenthis disclosure and anything incorporated by reference, this disclosureshall supersede.

Non-uniform Soiling

PV array modules undergo soiling as dust, dirt, or other contaminantsaccumulate on them. Under many conditions, soiling accumulates uniformlyacross the surfaces of the modules. However, under other conditions,soiling may accumulate non-uniformly on the modules. In many solar powerplants, modules are installed at a tilt angle to better face the sun. Inthis case a condition leading to non-uniform soiling commonly occurswhen water condensation on the modules rinses dust and dirt from thetops towards the bottoms of the modules, without completely cleaningthem. This leaves a band of soiling accumulation across the bottoms ofthe modules—referred to as “edge soiling”. The extent of the edgesoiling effect may depend on framing, tilt angle, and mountingmechanisms of the module, as these affect the pattern of water flowacross the module surface. Other conditions may also lead to non-uniformsoiling of various patterns, such conditions including wind effects;specific details of the module construction and/or mounting system;behavior of a tracking system which adjusts module orientation, if used;and/or other details of the site.

Uniform and non-uniform soiling have different effects on the electricaloutput characteristics of PV modules. This is illustrated in FIG. 3,which depicts simulated I-V curves of a hypothetical 72-cell, −300 W PVmodule under various conditions. Simulated I-V curves were generatedwith a commercial circuit modeling software package, according totechniques familiar to those skilled in the art. The module is taken tohave all 72 cells connected in series, with no bypass diodes. ConditionA represents the clean state of the module, while conditions B, C, and Drepresent various soiled states, with different shading resulting fromthe pattern of soiling. FIG. 3 shows the simulated I-V curve for eachcondition, as well as the short-circuit current (I_(sc)) and maximumpower (P_(max)) value corresponding to each curve. The maximum powerpoint of each curve is also indicated with an open circle. The reductionin either I_(sc) or P_(max) due to soiling is quantified by a soilingratio (SR), calculated as the ratio of the I_(sc) or P_(max) valuemeasured in the soiled state to that measured in the clean state. Thesevalues are denoted SR_(Isc) and SR_(Pmax), respectively on the FIGURE.In the present case we assume the module to be at the same temperaturein the clean and soiled state, and therefore neglect temperaturenormalization of the quantities, which should be considered in thegeneral case.

Condition B corresponds to uniform soiling leading to an equal 10%shading of all cells. In this case, I_(sc) and P_(max) are reduced to90% and 89.6% of their values for the clean state, respectively, and1-SR_(Isc) is a good approximation of the portion of power lost due tosoiling.

Condition C corresponds to a 10% shading of 9 of the cells, and isintended to represent an edge soiling condition, while condition Dcorresponds to a 10% shading of 1 cell, representing a localizedcontaminant on part of the module. Note that in condition D, the I-Vcurve shows a step between approximately 25 V and 28 V on the x-axisarising from the reverse biasing of the one shaded cell by the remainingnon-shaded cells at voltages less than about 25 V.

It is apparent from the tabulated values in FIG. 3 that the non-uniformsoiling conditions exhibit different reductions in I_(sc) versus P_(max)as compared with the clean state. In particular, conditions B and C,corresponding to uniform soiling and edge soiling, respectively, cannotbe distinguished on the basis of changes in the I_(sc) values, eventhough the effects on P_(max) values are very different. Accordingly,1-SR_(Pmax) is a much better metric of power lost due to soiling ascompared with 1-SR_(Isc).

The conditions illustrated in FIG. 3 are only representative examples.Details of module construction, including number and arrangement ofcells and any included bypass diodes, lead to different kinds ofbehavior under various patterns of soiling. In general, for spatiallyuniform soiling, the soiling-induced reductions in I_(sc) and P_(max)are similar, while for sufficiently non-uniform soiling they are not.This illustrates the benefit of measuring P_(max) values as a centralaspect of a system according to the disclosed subject matter.

System Overview

FIG. 4A and FIG. 4B depict two embodiments of the disclosed subjectmatter. In both embodiments, soiling is determined from measurementsperformed by the measurement and control unit 104 of the I_(sc) andP_(max) of soiled reference module 100, which is substantially identicalto modules comprising the PV array of the solar power plant and which ismounted in substantially identical manner. The measured I_(sc) andP_(max) must be normalized to account for both the soiled referencemodule 100 temperature and the incident solar irradiance. In oneembodiment, depicted in FIG. 4A, solar irradiance is measured using aclean reference module 101 substantially identical to the soiledreference module 100. The clean reference module 101 is cleaned eithermanually or by an automatic system. In another embodiment, depicted inFIG. 4B, solar irradiance is measured using a clean reference cell 102,which is relatively small compared to clean reference module 101 andwhich, in one embodiment, is automatically cleaned by wash unit 108 ofthe disclosed subject matter.

FIG. 5 depicts an embodiment of the type disclosed in FIG. 4B, which isnow discussed in greater detail.

In FIG. 5, the system comprises a soiled reference module 100 and aclean reference cell 102 coupled to a measurement and control unit 104,which are placed outdoors at the site of the actual or prospective solarpower plant.

The soiled reference module 100 and clean reference cell 102 are mountedon a mounting rack 106, substantially co-planar with each other andsubstantially co-planar with the modules of the actual or prospectivesolar power plant.

The clean reference cell 102 is periodically cleaned with an automatedsystem, which may include spraying with a cleaning fluid and/or cleaningwith mechanical action. In a representative embodiment, cleaning isperformed once per day. In one embodiment, a wash unit 108, comprising afluid reservoir and a pump, delivers cleaning fluid through cleaningfluid tubing 110 to a spray nozzle 112 housed within or above thereference cell 102 enclosure, such that the cleaning fluid removesaccumulated soiling from the clean reference cell 102. The wash unit 108is controlled by the measurement and control unit 104 via electricalconnections 114. In another embodiment, the system includes a mechanicaldevice (not shown), such as a wiper blade, which removes accumulatedsoiling from the clean reference cell 102 either with or without the aidof cleaning fluid.

The soiled reference module 100 is of substantially the same type and ismounted in substantially the same way as the modules used in the actualor prospective solar power plant, and is therefore allowed to accumulatesoiling in the same manner and at the same rate. Soiling-related lossesin the output power of the soiled reference module 100 are thereforedeemed representative of soiling-related power losses in the solar powerplant.

The soiled reference module 100 is electrically connected to themeasurement and control unit 104 via its module leads 116. In oneembodiment, a temperature sensor (not shown), such as a ResistanceTemperature Device (RTD), is attached to the back side of the soiledreference module 100 and connected to the measurement and control unit104 in order to provide for measuring the soiled reference module 100temperature.

The clean reference cell 102 is used to provide an independent measureof solar irradiance. It is electrically connected to the measurement andcontrol unit 104 via electrical connections 118 which also includesignals from a temperature measurement device, such as an RTD, inthermal contact with the PV-active portion of the clean reference cell102. In one embodiment, an optical filter is integrated within the cleanreference cell 102 in order to reduce its spectral response mismatchversus that of the soiled reference module 100; this is of particularbenefit when the underlying PV technologies of the clean reference cell102 and soiled reference module 100 are different.

The measurement and control unit 104 measures the electrical outputparameters and temperature of the soiled reference module 100 and thesolar irradiance detected by the clean reference cell 102. Thesequantities are analyzed, either by the measurement and control unit 104or by a remote computing system (not shown) in order to determine theexpected electrical output of the soiled reference module 100 at thegiven conditions of temperature and irradiance were the module to beclean, and therefrom to determine the fraction of output power from thesoiled reference module 100 that is lost due to soiling, or,equivalently, the fraction of power actually generated in the soiledstate compared to the power which could be generated in the clean state.

The measurement and control unit 104 is powered by AC or DC powerprovided, in one embodiment, through power entry conduit 120. In oneembodiment, the system includes a battery for backup power; in oneembodiment, the battery is located within the measurement and controlunit 104. In another embodiment, the system is powered by one or more ofthe reference devices (100, 101, and/or 102) through a process in whichthe measurement and control unit 104 harvests power from the referencedevices (100, 101, and/or 102) in between measurements and stores energyfor use during measurements; for example, harvested energy may be storedin a battery located within measurement and control unit 104.

The measurement and control unit 104 communicates with remote computing,data logging, and/or control systems via a wired or wirelesscommunication method. In one embodiment, communication is via anEthernet cable, which, in one embodiment, is passed throughcommunication conduit 122.

In one embodiment, the enclosures of the elements depicted in FIG. 5 areweather-resistant for prolonged outdoor use.

In alternative embodiments, any of the elements described may becombined within a lesser or greater number of separate enclosures thandepicted in FIG. 5. In one embodiment, the clean reference cell 102 andthe measurement and control unit 104 are combined into a single unit. Inanother embodiment, these are further combined with the wash unit 108.

Measured Electrical Output

In one embodiment, the measured electrical output parameter of thesoiled reference module 100 is the short-circuit current (I_(sc)), fromwhich, following temperature correction, the output maximum power(P_(max)) of the soiled reference module 100 is estimated using knownparameters of the soiled reference module 100.

In another embodiment, the measured electrical output parameters of thesoiled reference module 100 may include, in addition to theshort-circuit current (I_(sc)), any of the maximum power (P_(max)),open-circuit voltage (V_(oc)), voltage at maximum power point, currentat maximum power point, or the entire current-voltage relationship (“I-Vcurve”). To measure these additional parameters, the measurement andcontrol unit 104 performs a sweep during which the I-V curve is measuredand then analyzes the I-V curve. This analysis could also be performedon a remote computing system.

Any of the measured parameters may be corrected for temperature and/orirradiance, using methods known in the art and based on storedconstants.

By measuring the I-V curve, the actual maximum power of the soiledreference module 100 may be directly measured, rather than estimatedfrom the short-circuit current.

When soiling is uniformly distributed across the soiled reference module100, estimation of the maximum power from the short-circuit current maybe more accurate than direct measurement, due to the higher measurementuncertainties associated with measurement and temperature-correction ofthe maximum power.

However, when soiling is non-uniformly distributed, estimation of themaximum power from the short-circuit current may be very inaccurate, asdiscussed above.

In one embodiment, both the short-circuit current and maximum power areused to determine soiling-related power losses, and software within themeasurement and control unit 104, or within a remote computing device,determines the most accurate result. For example, in one embodiment, thesoiling-related power loss is determined exclusively from the measuredmaximum power whenever the difference between this loss and thatdetermined exclusively from the short-circuit current exceeds thedifference in uncertainty between the two measurements, or whenever thedifference in uncertainty is negligible.

In one embodiment, the system uses the measurements of bothshort-circuit current and maximum power to report a metric whichquantifies the degree of non-uniformity of the soiling. Under certainsituations this metric may indicate actionable problems with the solarpower plant.

In one embodiment, the measurement and control unit 104 holds the soiledreference module 100 in a designated electrical state in betweenmeasurements in order to prevent or reduce long-term degradation orother performance changes of the soiled reference module 100 that mayoccur in certain electrical states. The designated electrical state mayinclude short-circuit, open-circuit, maximum power, or others. Forexample, crystalline silicon PV modules may be held at open-circuit inbetween measurements, in order to reduce degradation due to hot-spotsthat may occur for prolonged operation at short-circuit.

Measurement Circuit

The measurement and control unit 104 contains measurement channels forthe soiled reference module 100 and either the clean reference module101 or clean reference cell 102. In one embodiment, the measurementchannel for the soiled reference module 100 measures any of itscharacteristic I-V parameters as described above, includingshort-circuit current and maximum power. In one embodiment, themeasurement channel contains a sweep circuit comprising a transistor inseries with the soiled reference module 100, wherein the transistor maybe controlled to moderate and sweep the current flowing through thesoiled reference module 100 while its current and voltage are measuredby the measurement channel, thereby collecting an I-V curve. Similarly,to measure irradiance using the clean control device, in one embodiment,a sweep circuit measures the I-V curve of the clean reference module101, or, in another embodiment, of the clean reference cell 102. Inother embodiments, a sweep circuit measures only the short-circuitcurrent of a clean reference module 101 or clean reference cell 102,without sweeping its I-V curve, by maintaining the sweep circuit in theshort-circuit condition.

The I-V curve sweep time must be sufficiently short to prevent excessiveheating of the sweep circuit during the I-V sweep (due to received powerfrom the soiled reference module 100 or clean reference module 101);sufficiently short to minimize the impact of irradiance changes duringthe sweep; sufficiently long to allow accurate measurement of currentand voltage during the sweep; and sufficiently long so that the soiledreference module 100 or clean reference module 101 capacitance does notaffect the measured current and voltage data. Sweep times on the orderof 100 milliseconds to 1 second would typically meet these requirements;however, other times may also be employed and remain within the scope ofthis disclosure.

In one embodiment, the sweep circuit may progress either fromshort-circuit to open-circuit or vice versa. In one embodiment, inbetween measurements the circuit may be held in either the short-circuitor open-circuit condition.

FIG. 6 depicts one embodiment of an I-V curve sweep and extraction ofparameters from the measured I-V data. In FIG. 6, the sweep is assumedto progress from a short-circuit condition to an open-circuit conditionof the soiled reference module 100. Curves 202 and 204 depict themeasured current and voltage versus time during the sweep, while curve206 depicts the power calculated from the measured current and voltagecurves 202 and 204. The sweep is divided into three periods,corresponding to an initial low impedance state 220, a transition state222, and a high impedance state 224 of the sweep circuit. During theinitial low-impedance state 220, which lasts on the order of 10-100 msfollowing triggering of the sweep circuit, the sweep circuit maintainsthe module at the short-circuit condition, and, in one embodiment, theI_(sc) value is measured during an initial period 210. In the transitionstate 222, which lasts on the order of 100-500 ms, the voltage is rampedupwards towards a maximum value indicated as 216, causing the current202 to fall. During the final high-impedance state 224, which lasts onthe order of 10-100 ms, the sweep circuit forces the soiled referencemodule 100 to its open-circuit condition and, in one embodiment, theV_(oc) value is measured during a final time period 212. The P_(max)value 214 is calculated by analysis of the calculated power curve 206,and, in one embodiment, the I_(sc) and/or V_(oc) values are determinedby analysis of the measured current 202 versus voltage 204 curves. Inone embodiment, the ramp rate of the voltage curve 204, or equivalently,the IV Sweep V_(max) value 216, are programmable and controlled bysoftware within the measurement and control unit 104. Typically, thesweep would be set so that the voltage curve 204 reaches the soiledreference module 100 Voc value just before the onset of thehigh-impedance state 224, stretching the measured I-V curves to fill thetransition state region 222 for optimal data acquisition quality. FIG. 6illustrates an alternate (undesirable) condition where the voltage curve204 does not reach all the way to the Voc value before the onset of thehigh-impedance state 224, whereupon the sweep circuit cuts off thesoiled reference module 100 current to end the sweep. In one embodiment,the direction of the sweep could also be reversed. In one embodiment,the sweep always ends in either the high-impedance or low-impedancestate, thereby minimizing power dissipation in the sweep circuit. Theexample is given for measurement of the soiled reference module 100, butapplies equally to the clean reference module 101 or clean referencecell 102. Although specific time periods are provided for reference,other time periods could be employed and remain within the scope of thisdisclosure.

FIG. 7 depicts one embodiment of an implementation of the sweep circuit.A sweep generator circuit 252 responds to a control signal 250, e.g.from a microcontroller, and produces a low-voltage ramp signal 253. Thevoltage of the ramp signal 253 is used to program a feedback circuit 256which controls one or more transistor elements 258 connected in seriesbetween the input terminals 260 and 261 and a sense resistor R5 of acurrent measurement circuit. Initially the ramp signal 253 is saturatedat one supply rail of U2, causing the sweep circuit to remain in thelow-impedance state 220. Following an integration period, the sweepcircuit enters the transition state 222, during which the feedbackcircuit 256 controls the transistor elements 958 in order to maintainthe voltage at 960 equal to the low voltage ramp signal 253 times a gainfactor set by R8 and R9. Finally the limit circuit 254 detects theendpoint of the low voltage ramp signal 253 and forces the sweep circuitinto the high impedance state 224 by raising the gain of the feedbackcircuit 256. In one embodiment, the gain of the feedback circuit 256 isprogrammable from a microcontroller, e.g. by replacing R9 with aprogrammable digital potentiometer integrated circuit. In oneembodiment, the sweep time can be altered by additional controls thatmodify the ramp rate of the sweep generator circuit 252 or bypass theramp signal 253. In one embodiment, the sweep circuit containsadditional elements (not shown) for frequency compensation and amplifierstability.

Mounting of Reference Devices

In one embodiment the mounting rack 106 is a separate mounting structurededicated to the measuring system, as depicted in the embodiment of FIG.5, while in another embodiment the mounting rack 106 is a part of themounting structure for the power-producing modules of the solar powerplant. In one embodiment the mounting rack 106 holds the pair ofreference devices (100 and 101, or 100 and 102) in a fixed position,while in another embodiment, typically when the mounting rack 106 ispart of the mounting structure for the power-producing modules of thesolar power plant, it rotates about one or more axes in order to trackthe position of the sun throughout the day.

In order to minimize measurement errors, the pair of reference devices(100 and 101, or 100 and 102) should be held coplanar with each otherpreferably within less than 0.5 degrees.

In one embodiment, the system includes mechanical mounting features thatensure such alignment.

In another embodiment, the system includes adjustment mechanisms toenable the mounting angles to be calibrated. For example, in oneembodiment, clean reference cell 102 is provided with one or morealignment screws 430, in alignment screw brackets 432, which pressagainst a mounting plate 432 in order to adjust the alignment. In oneembodiment, two such alignment screws are provided in order to adjustthe alignment along two axes, including an azimuthal and a tilt angle.

In another embodiment, software operating within the measurement andcontrol unit 104 or within a remote computing system corrects themeasurements for the effect of angular differences.

In one embodiment, the effect of any residual angular alignmentdifferences between the soiled reference module 100 and clean referencemodule 101 or clean reference cell 102 is reduced by averaging themeasured readings of soiling-related power loss throughout the course ofa day. For example, in the case where the pair of reference devices (100and 101, or 100 and 102) have a slight difference in azimuthalalignment, a bias error will occur where one of the pair of referencedevices (100 and 101, or 100 and 102) receives more irradiance than theother in the morning and less in the afternoon. The effect of this errormay be greatly reduced by averaging equal contributions of readings frombefore and after the local solar noon time of each day.

Wash Unit

FIG. 8 depicts an embodiment of the wash unit 108 intended to be usedfor automated cleaning of the clean reference cell 102.

The wash unit 108 is housed in the wash unit enclosure 302 and comprisesa cleaning fluid reservoir 304, cleaning fluid 306, and pump 308 fordelivering the cleaning fluid 306 from the fluid intake tubing 310 tothe spray nozzle 112 via cleaning fluid tubing 110.

In one embodiment the cleaning fluid 306 is water.

In another embodiment, the cleaning fluid 306 is a solution containinganti-freeze properties allowing operation and storage in sub-freezingtemperatures, for example down to −25.degree. C. Suitable cleaningfluids, include, for example, automobile windshield washing fluids,especially those formulated to be environmentally benign.

In one embodiment, a collection tray 124 is provided underneath theclean reference cell 102 to collect cleaning fluid 306 and prevent itfrom spilling onto the ground and/or to recapture the cleaning fluid 306and direct it back into the cleaning fluid reservoir 306 for re-use.

In one embodiment, the cleaning fluid reservoir 304 of the wash unit 108contains a fluid level sensor 322 that indicates when the cleaning fluidreservoir 304 is nearly empty. The measurement and control unit 104senses low fluid levels and reports the need for refilling the cleaningfluid reservoir 304.

In one embodiment, the wash unit 108 includes a flow sensor 312 formeasuring the flow rate of cleaning fluid 306 during operation of thewash unit 108. The flow rate is sensed by the measurement and controlunit 104 in order to determine whether flow is within acceptable limitsand thereby identify faults with the system. In another embodiment, aflow sensor 313 is located near the discharge point of the fluid intothe spray nozzle 112, within the enclosure housing the clean referencecell 102, thus providing the additional ability to identify any leaks orblockages between the wash unit 108 and the clean reference cell 102.

In one embodiment, the flow rate is measured in order to determine thevolume of cleaning fluid 306 dispensed and to operate the wash unit 108until a predetermined fluid volume has been dispensed, ensuringrepeatable cleaning and minimal use of cleaning fluid 306. This mayparticularly be required in situations where height difference betweenthe clean reference cell 102 and the wash unit 108 is variable (e.g.when the reference devices 100,102 are mounted on a tracking system),leading to variable pressure within the cleaning fluid tubing 110.

In one embodiment, the temperature of the clean reference cell 102 ismeasured during operation of the wash unit 108 in order to determinewhether the cleaning system is operating correctly. Under sunnyconditions, application of cleaning fluid 306 to the clean referencecell 102 should result in a momentary decrease in temperature. Themeasurement and control unit 104 detects whether the temperature of theclean reference cell 102 decreases in the expected manner, thus allowingpotential identification of faults such as leaks, blockages, ormis-direction of the spray nozzle 112.

In one embodiment, the wash unit 108 is provided with insulation 326and/or with at least one heating element 324 to prevent freezing of thecleaning fluid 306 in cold conditions. The heating element 324 may becontrolled by measurement and control unit 104.

Reference Cell

FIG. 9 depicts one embodiment of the clean reference cell 102 that hasspecial features for use within a system of the disclosed subjectmatter, beyond those of a PV reference cell of typical construction.

The clean reference cell 102 is housed in reference cell enclosure 402.An encapsulated PV reference cell 404, comprising a cell of photovoltaicmaterial bonded within an encapsulant material and further bonded to acover glass 406, in one embodiment, is mounted within the reference cellenclosure 402. The mounting mechanism provides a temporary or permanentseal which prevents moisture entry to the reference cell enclosure 402.In one embodiment, a temperature sensor 410, such as a resistivetemperature detector, is fixed in thermal contact with the back side ofthe encapsulated PV reference cell 404.

In one embodiment an optical filter is integrated within the referencecell enclosure 402 or used in place of cover glass 406, in order toadjust the spectral response of the encapsulated PV reference cell 404to provide better spectral response matching with soiled referencemodule 100.

A spray nozzle 112 directs cleaning fluid 306 from cleaning fluid tubing110 onto the cover glass 406 of the encapsulated PV reference cell 404,providing fluid spray 420 which cleans the cover glass 406.

In one embodiment, spray nozzle 112 is mounted on reference cellenclosure 402 and cleaning fluid tubing 110 passes cleaning fluid 306through the reference cell enclosure 402. In another embodiment, themounting of spray nozzle 112 is not integral to reference cell enclosure402 but comprises, for example, a separate bracket which holds spraynozzle 112 near reference cell enclosure 402, such that cleaning fluidtubing 110 need not be connected to reference cell enclosure 402.

In one embodiment, a flow sensor 313 included within or near thereference cell enclosure 402 measures the flow rate of cleaning fluid306 to the spray nozzle 112. The flow rate is measured in order that thesystem may dispense a minimally sufficient quantity of cleaning fluid306 and/or to ensure proper operation of the system and absence ofleaks, as discussed above.

In one embodiment, reference cell enclosure 402 contains one or moreheating elements 412 which heat the reference cell enclosure 402. In oneembodiment, heating elements 412 are mounted to the inside top side ofthe reference cell enclosure 402, to preferentially heat the top side.

In one embodiment, reference cell enclosure 402 contains a circuit board416 through which connections to each of the enclosed electricalelements are provided, as depicted in FIG. 5.

In one embodiment, the clean reference cell 102 is provided with one ormore alignment screws 430, in alignment screw brackets 432, which pressagainst a mounting plate 432 in order to adjust the alignment. In oneembodiment, two such alignment screws 432 are provided in order toadjust the alignment along two axes, including an azimuthal and a tiltangle.

Measurement of Snow Losses

The accumulation of snow, ice, and frost on modules within a solar powerplant produces power losses in a manner similar to that associated withthe accumulation of soiling, by preventing solar irradiance fromreaching all or portions of the modules. Such snow-related (includingice- and frost-related) power losses may also be measured by the systemof the disclosed subject matter, provided that the clean reference cell102 is kept free of accumulated snow, ice, and frost in order that itmay accurately measure the incident solar irradiance.

In one embodiment, the reference cell enclosure 402 is heated by heatingelements 412 to melt snow, ice, and/or frost accumulated on top of thereference cell enclosure 402, causing the snow, ice, and/or frost toevaporate and/or slide off. In one embodiment, heating elements 412 areactivated based on readings of the temperature of the clean referencecell 102 measured with temperature sensor 410. In another embodiment,heating elements 412 are activated based on externally or remotelymeteorological data that indicate the likely presence of snow, ice, orfrost.

In one embodiment, the wash unit 108 is used to partially or completelyremove accumulated snow, ice, or frost from the clean reference cell102. In one embodiment, this is done by spraying cleaning fluid 306 withanti-freeze properties on the clean reference cell 102. In oneembodiment, the cleaning fluid 306 may be heated.

In one embodiment, a gap (not depicted in FIG. 5) is providedsurrounding the clean reference cell 102, such that any snow sliding offof adjacent surfaces (e.g. the soiled reference module 100) does notslide onto the clean reference cell 102.

Calibration

Accurate determination of soiling-related power losses by comparison ofthe outputs of the reference devices (100 and 101 or 100 and 102) mayrequire that both the reference devices (100 and 101 or 100 and 102) andthe measurement and control unit 104 be calibrated.

In one embodiment, both the clean reference cell 102 and the measurementand control unit 104 are calibrated in a laboratory or manufacturingenvironment prior to delivery to the installation site and subsequentlyrecalibrated as needed or at periodic intervals either in a laboratoryor at the installation site.

In one embodiment, the soiled reference module 100 and/or cleanreference module 101 are also calibrated in a laboratory environmentprior to delivery to the installation site.

However, laboratory calibration results in significant expenses relatedto removing modules from service, shipping them to and from calibrationlaboratories, and re-installing them into service. Therefore, in anotherembodiment, the soiled reference module 100 and/or clean referencemodule 101 are calibrated and/or recalibrated at the installation siteusing portable equipment. In another embodiment, the soiled referencemodule 100 and/or clean reference module 101 are automaticallycalibrated by the system of the disclosed subject matter. In thisembodiment, when both the soiled reference module 100 and the cleanreference module 101 or clean reference cell 102 are known to be in aclean state, e.g. after cleaning by personnel or after rain events,measurements performed by the measurement and control unit 104 are usedto determine either relative or absolute calibration constants, withanalysis being performed either within the measurement and control unit104 or within a remote computing device.

Data Analysis

In one embodiment, measured data are analyzed to filter out measurementsperformed during periods of rapidly changing irradiance, e.g. due topassing clouds affecting the clean reference cell 102 and soiledreference module 100 differently at an instant in time.

In one embodiment, measured data are integrated or averaged overportions of the day including approximately equal contributions ofmeasurements both before and after solar noon, in order to minimizeerrors associated with angular alignment differences between thereference devices (100 and 101 or 100 and 102).

In one embodiment, measured data are analyzed to determine rates ofchange over time of soiling-related losses, in addition to or instead ofdetermining soiling-related power losses.

Multiple Devices

In one embodiment, the system contains multiple soiled reference modules100, and a soiling-related power loss is determined for each.

In one embodiment, the system contains multiple clean reference modules101 or clean reference cells 102.

In one embodiment, multiple measurement and control units 104 arenetworked together, the entire system thereby measuring multiplereference devices (100, 101, and/or 102).

Irradiance Measurement

In one embodiment, the system is used for the purpose of irradiancemeasurement, in addition to or instead of for the purpose of measuringsoiling-related power losses, and the soiled reference module 100 may beomitted. In one embodiment, the system comprises an irradiance sensor,which may comprise either a clean reference cell 102 or a pyranometerwhich can take the place of the clean reference cell 102. The irradiancesensor is cleaned with the automatic cleaning unit.

Solar Noon

“Solar noon” as used in this disclosure is with reference to thesubstantially co-planar soiled reference module 100 and clean referencecell 102, rather than to clocks or compass coordinates. In other words,“solar noon” as used in this disclosure means the time of each day whenthe solar angle of incidence upon the soiled reference module 100 andclean reference cell 102 is minimized, using a definition of angle ofincidence as measured from the normal.

Modules and Cells

In this disclosure, any use of the word “module” may be interchangedwith the word “cell”, and vice-versa.

Clean Reference

The clean reference cell 102 may be maintained in a clean state by anysuitable means including, for example, manual cleaning or simplycovering it when not in use to keep it clean and then uncovering it whenneeded for measurement, whether said covering and uncovering isperformed manually by personnel or automatically by equipment.

Combinations of Elements

As stated above, any of the disclosed elements may be combined in agreater or lesser number of enclosures and remain within the scope ofthis disclosure. For example, a clean reference cell 102 may be combinedin a single enclosure with a soiled reference cell (replacing soiledreference module 100).

CONCLUSION

Although example diagrams to implement the elements of the disclosedsubject matter have been provided, one skilled in the art, using thisdisclosure, could develop additional hardware and/or software topractice the disclosed subject matter and each is intended to beincluded herein.

In addition to the above described embodiments, those skilled in the artwill appreciate that this disclosure has application in a variety ofarts and situations and this disclosure is intended to include the same.

While the invention has been described with respect to a limited numberof embodiments, the specific features of one embodiment should not beattributed to other embodiments of the invention. No single embodimentis representative of all aspects of the inventions. Moreover, variationsand modifications there from exist. For example, the invention describedherein may comprise other components. Various additives may also be usedto further enhance one or more properties. In some embodiments, theinventions are substantially free of any additive not specificallyenumerated herein. Some embodiments of the invention described hereinconsist of or consist essentially of the enumerated components. Inaddition, some embodiments of the methods described herein consist of orconsist essentially of the enumerated steps. The appended claims intendto cover all such variations and modifications as falling within thescope of the invention.

What is claimed is:
 1. A system for measuring electrical output loss ina photovoltaic array due to soiling, comprising: a pair of photovoltaicreference devices placed substantially proximate to the photovoltaicarray and substantially co-planar to modules forming the array, saidpair of photovoltaic reference devices comprising: a first referencedevice, designated as a soiled reference device, comprising aphotovoltaic module or cell representative of a portion of thephotovoltaic array, wherein the soiled reference device is allowed toaccumulate soiling at a rate representative of the rate of soiling ofthe photovoltaic array; a second reference device, designated as a cleanreference device, comprising a photovoltaic module or cell, wherein asensing surface of said clean reference device is maintainedsubstantially clean and free of accumulated soiling; a measurement unitfor performing measurements of a subpart, wherein said subpart comprisessaid soiled reference device, said clean reference device, andcombinations thereof; wherein said measurements of said soiled referencedevice comprise measurements of a first short-circuit current; andwherein said measurements of said clean reference device comprisemeasurements of a second short-circuit current; and a computing device,wherein said computing device analyzes said measurements associated withsaid subparts for determining a soiling impact analysis.
 2. The systemof claim 1, wherein said soiling impact analysis comprises calculationof a soiling ratio, wherein said soiling ratio further comprises a ratioof an actual output of said soiled reference device to an expectedoutput of said soiled reference device when clean.
 3. The system ofclaim 1, wherein said soiling impact analysis comprises calculation of asoiling loss, wherein said soiling loss is a difference between saidactual output of said soiled reference device and said expected outputof said soiled reference device when clean, and wherein said soilingloss may be normalized by said expected output so that said soiling lossis expressed as a fractional loss.
 4. The system of claim 1, whereinsaid measurement unit further comprises a temperature sensor formeasuring a temperature of said subpart, wherein said measurementcomprises a measurement from a group consisting essentially oftemperature of said subpart, a temperature difference across saidsubpart, and combinations thereof.
 5. The system of claim 4, whereinsaid temperature measurements are used to temperature-correct saidmeasurements of said subparts.
 6. The system of claim 1, wherein saidmeasurement unit maintains said subpart in a designated electrical statebetween performance of said measurements, wherein the designatedelectrical state comprises essentially a short-circuit state, anopen-circuit state, or a maximum power state.
 7. The system of claim 6,wherein said designated electrical state is chosen to prevent orsuppress degradation of said subpart.
 8. The system of claim 1, whereinsaid computing device stores a difference in angular alignment betweensaid subparts or between said subparts and said photovoltaic array, andwherein said computing device corrects said measurements for saiddifferences in angular alignment.
 9. The system of claim 1, wherein saidsoiling impact analysis is calculated by averaging said measurements ofsaid subparts in order to calculate a daily-average value of saidsoiling impact analysis.
 10. The system of claim 9, wherein saidaveraging is performed over a restricted time period of each day,including substantially equal contributions of measurements before andafter the time of minimum solar angle of incidence of each day.
 11. Thesystem of claim 9, wherein said measurements are filtered, prior toaveraging, to remove values corresponding to low irradiance, rapidlychanging irradiance, or other conditions that degrade measurementquality.
 12. The system of claim 1, wherein said subpart supplies powerto said measurement unit between performances of said measurements. 13.The system of claim 1, wherein said computing device utilizes saidmeasurements of said clean reference device to calibrate saidmeasurements of said soiled reference device during a period in whichsaid soiled reference device is known to be substantially clean.
 14. Thesystem of claim 13, wherein said calibration establishes a baselinevalue for said soiling impact analysis.
 15. The system of claim 1,wherein said clean reference device and said soiled reference device arehoused within a single enclosure.
 16. The system of claim 1, whereinsaid clean reference device is maintained in said substantially cleanstate by protecting it with a cover to prevent contamination in betweenmeasurements, and wherein for said measurements said clean referencedevice is removed from said cover, or said cover is removed from saidclean reference device, either manually by personnel or automatically byequipment.
 17. The system of claim 1, wherein said system furthercomprises an automatic cleaning unit, wherein said automatic cleaningunit maintains a soiling free surface of said clean reference device.18. The system of claim 17, wherein said automatic cleaning unit uses apressurized gas stream directed at said clean reference device to removeaccumulated soiling.
 19. The system of claim 17, wherein said automaticcleaning unit comprises a cleaning fluid reservoir and a pump fordirecting said cleaning fluid through a spray nozzle onto a coversurface of said clean reference device for removing accumulated soiling.20. The system of claim 19, additionally comprising a collection traywhich captures dispensed cleaning fluid to prevent spilling and/or forre-use in said cleaning fluid reservoir.
 21. The system of claim 19,wherein said cleaning fluid resists freezing.
 22. The system of claim19, wherein said automatic cleaning unit further comprises insulation, aheater, or combination thereof, for preventing freezing of said cleaningfluid.
 23. The system of claim 19, wherein a housing of said cleanreference device incorporates said spray nozzle.
 24. The system of claim19, additionally comprising a flow sensor for measuring a flow rate ofsaid cleaning fluid, wherein dispensing of said cleaning fluid continuesuntil either an integral of said flow rate over time reaches apredetermined limit, or said flow rate is out of a normal range.
 25. Thesystem of claim 1, further comprising a heater for removing anaccumulation of frozen precipitation from a surface of said cleanreference device, and wherein according to said measurements of saidclean reference device, said system determines an electrical output lossvalue for the soiled reference device resulting from said frozenprecipitation.
 26. The system of claim 25, wherein an enclosure of saidclean reference device incorporates said heater.
 27. The system of claim1, wherein said clean reference device incorporates an optical filter toreduce its spectral response mismatch versus that of said soiledreference device.